Process of working steel for preventing surface defects thereof



United States Patent'f PROCESS OF WORKING STEEL FOR PREVENTING SURFACE DEFECTS THEREOF Tadashi 0htake, Tobata City, and Keizo Ishizaki and Naoki Eguchi, Yawata City, Japan, assignors to Yawata Iron and Steel Co., Ltd., Tokyo, Japan NoDrawing. Filed Feb. 12,1957, Ser. No. 639,633

Claims priority, application Japan Feb. 13, 1956 17 Claims. (Cl. 29'528) The present invention relates to the process of treating metal for preventing surface defects, particularly for treating steel ingots or products for the purpose of preventing surface defects occurring on the surface thereof when they are subjected to hot rolling or working. fOne object of our invention is to provide an improvedprocess of producing steel products of superior quality in which "surface defects tending to occur on the surface thereof are eliminated by the process of our invention.

Another object of our invent-ion is to provide an easy and effective process of hot working steel products in order to prevent them from incurring surf-ace defects, such as, seam and crack, thereon when they are hot worked.

Heretofore surface defects have often occurred on the surface of steel when subjected to hot working, such as, rolling, forging, drawing, and extruding. Such surface defects result in many deleterious failures in the finished steel products so that the value of products is loweredas well as the quality of steel is considerably decreased.

Heretofore many articles on the cause of formation of such surface defects have been published, the details of which will be hereinafter described, but, to make them in short, the cause is described as follows: when steel is heated prior to hot working, the iron content on the surface thereof is selectively oxidized in the hot atmosphere to form scale; consequently, a small quantity of undesirable elements, such as, Cu, Sn, and As which have less ionization than Fe remain in the metal texture immediately below the scale in concentrated form; such elements in concentrated form are sometimes converted into an alloy in molten form to disperse itself into the interior of steel through the grain boundary from the skin thereof so that the strength of steel is deteriorated. Therefore it is considered that the skin of steel is easily ruptured by the force imparted by the working process to form a tear or crack thereon.

On the other hand, steel contains somewhat small amounts of undesirable elements. hereinabove mentioned depending upon ores and other raw materials and what not. Accordingly, it is inevitable for any grade of steel.

to have some seam or crack on its surface dependent on the heating step. Therefore, when the finished surface of steel products'is particularly taken into consideration,-

or when the content of such undesirable elements (including the addition of favorable elements for the particular purpose, for example, the addition of copper for the improvement of corrosion resistance,-or precipitation aging-property) is relatively large, it has been heretoforeproposed to coat the surface of steel with a coating substance on heating it prior to workings, thus insulating steel from the hot atmosphere to diminishor eliminate the formation of scale on the surface thereof in order to eliminate the formation of an alloy in concentrated enriched liquid phase, or to add a suitableamountof nickel to steel to raise. the melting point of the alloy-like substance in concentrated form produced on the surface of steel texture immediately below the scale thereof with a view to maintaining the concentrated alloy-like substance in solid phase at the working temperature of steel so that it is made to be inactive for the formation of surface defects. However, the advantage of the'former process is unstable and particularly undesirable in the point of heat balance at a heating step, and the latter process is not generally adopted for commercial practice, because nickel to be added is not only expensive but also almost the same amount of copper as contained in steel is required, so that it does not pay from an economical point of view, except a particular case.

After ourexhaustive study of experiment, we inventors have ascertained the above-mentioned observation of defect formation on the surface of steel that small amounts of undesirable elements, such as, copper, tin, and arsenic,

which are of less ionization than iron contained in the steel, are isolated and remain as such in concentrated enriched form Without being oxidized while iron element is selectively oxidized by the action of hot atmosphere and converted into scale so that they produce alloy like substances in liquid phase.

In addition we have discovered that the formation of surface defects, such as, rupture or crack, is not brought about by the deterioration of steel due to the dispersion of such alloy-like substances in liquid phase through the grain boundary into the'interior of steel from its skin and the ready'se'paration of grain boundary by the force subjected by the working step, but, on the contrary, sur face defects are formed by the instantaneous attack'and destruction of grain boundary with the co-operation of stress caused by the working step due to the strong chemical affinity of such metallic substances inliquid phase for steel, and the extent of surface defects produced compound as a treating agent, thereby our invention ofa' novel and effective processfor hot working metal in order to prevent surface defects thereof has been invented. 1A treating agent for .use in our invention can'; be selected from-either a sulphide compound of iron,- inanganese, sodium, potassium, and aluminum or the mixture:

thereof, and a mineral containing at least one compound selected from the above-mentioned substances-in its major; constituent, and the mixed mineral, and the" mechanical ly mixed. mineralscontaining at'leastone sulphide and; iron oxide (including mill scale) ora" mineral containing? iron oxide in its main constituent, in other wordsfi-a;

compound containi'ngat least -'o'rie"sulpl3 ic 1e selected" from the abovementioned'compounds.

In accordance with our invention, the surface of steel to be worked is treated with the treating agent for preventing surface defects from formation prior to hot working, and in practice, the following manner of treatment may be adopted singly or in combination:

(1) Application of treating agent in liquid form by melting:

(a) Immerse steel to be worked in the molten bath of treating agent prior to either working or at a suitable period during working, then subject it to working.

(5) Apply the molten agent on steel to be worked prior to either working or at an appropriate period during working, then subject it to working.

(2) Application of treating agent in powder form of suitable size:

(0) Spray a suitable quantity of powdered treating agent on steel to be worked in the heating furnace prior to working operation, then subject it to working after having melted'at least a part of the agent.

(d) Spray a suitable quantity of powdered treating agent on steel to be worked prior to either working or at a suitable period during working outside the heating furnace, then reheat it in the furnace, then work it after having melted at least a part of agent.

(e) Spray a suitable quantity of powdered agent on steel to be worked prior to either working or at a suitable period during working outside the heating furnace, then work it after having melted a part of the agent either by the inherent heat retained by steel itself or a supplemental external heat.

In any process of the above-mentioned, it is advantageous to remove the scale formed by the heat on the surface of steel to be worked by an appropriate means before applying the agent. However, the removal of scale is not an indispensable step, because the melting point of the scale decreases owingto the formation of molten substance with the agent at an elevated temperature of hot working, andthe molten substance reacts also with the metallic substance in liquid phase, thereby the preventionof surface defects is effected.

In reference to the suitable amount of treating agent described in Process 2, the suitable amount of the agent'indicates that the content of sulphide contained in the agent is sufiicient to inactivate or absorb metallic substances in liquid phase in concentrated enriched form immediately below the scale. It is preferable that the quantity of sulphide by weight sprayed per unit area should be almost the same quantity by weightv of enriched metallic substances in liquid; phase expected to be present thereon per unit area. It is of course to be understood that the absolute value of the quantity should depend entirely upon the, chemicalcompositionof steel to be worked and the heat course thereof;

Referring to the suitable period in the course of working described in Processes 1-2, it means the period in the course of or after a relatively light working operation prior to subjecting steel to heavy working, for example, in practice, it means a scale breaking stepv in the course of rolling.

As we have hereinabove described our novel process, the surface defect or seam. or crack which usually occurs onthe surface of steel at the time of hot working is brought about by the reaction of iron produced in concentrated enriched form on the skin of steel texture immediately below the scale and metallic alloy substances in liquid phase having a strong atfinity for said iron under heat. Metallic alloy substances in liquid phase consistof the major constituents. comprising copper, tin, and arsenic. Accordingly, theobjcct of preventing-the formation of surface defects may beattained by eliminating metallic substances in. liquid. phasetzthrough' some 4. means or by chemically inactivating said metallic substances. Any treating agent of the above-mentioned composition containing sulphide in its major constituent is able to answer the purpose of our invention. For example, the mixture of iron sulphide and iron oxide is applied to the surface of hot steel to be worked, then the mixture melts down so readily (the melting point of the mixture ranges 940l 170 C. in general, the temperature being lowered further with the addition of sodium sulphide) that it immediately reaches the surface of steel with the formation of molten metallic substances together with a thin scale covered on the surface thereof to make inert the molten metallic substances having the possibility of producing surface defects by the reaction of copper, tin, and arsenic with the treating mixture. The reaction is considered as follows:

The reaction proceeds to the right to equilibrium at the elevated temperature. Accordingly, the larger part of copper in enriched form is converted to copper sulphide which does no harm. A part of remaining copper forms matt on dissolving readily into iron sulphide with copper sulphide so that it is absorbed into the treating agent on the surface of steel and carried away. Therefore the surface defects would never occur after the treatment, no matter how heavy working is imposed on steel. Furthermore, the role of iron oxide added to the treating agent is to lower the melting point of the major con stituent, sulphide, thereby the reaction of molten alloy substances in concentrated form and sulphide is facilitated.

The embodiments of our invention are described hereinafter. in addition, in Examples 1-3, the results of our treatment are shown in the decrease of occurrence of surface defects on the surface of hot impact bend test sample of steel, which test is generally adopted as a test for the examination of occurrence of surface defects on steel at hot working.

Example I Inreference to the hot impact bend test of a commercial steel containing copper 0.18%, tin 0.05%, and arsenic' 0.06%, heated in the atmosphere, the results obtained by our invention for preventing surface defects on the outer surface of bent sample are as follows:

Treatment Occurrence of Defects mean depth oi defect, mm.

Heat sample 1,200 O. in the atmos- 0.05

phere min., then bend. (nontreated).

Spray mixture of powdered FeO and FeS 0. 05

(containing S 18%) on sample, melt mixt, then bend. (Process 2).

Dip sample in the bath of mixed FeO and FeS (containing S 18%) 10 see, then hand. (Process 1).

Heat sample l,200 O. in the atmos- 0. 40

phere 2 hrs., then hand. (nontreated).

Spray powdered iron sulphide ore (con- 0. 20

taming S 3346%) on sample, heat 90 see, their band. (Process 2).

Spray mixture. of powdered iron sul- 0.15

phide ore 60% and mill scale 40% on sample, heat 90 sec., than bend.

many.

almost none.

none none.

many.

decrease, but

remain here and there.

very few.

almost none.

Example '2 Referring to the hot impact bend test of three grades of steel containing copper 0.68%, 1.08%, and 2.0%, respectively, and Sn 0.02%, heated in the atmosphere, the results obtained by our invention with a view to preventing surface defects on the outer surface of bent sample are as follows:

Treatment Occurrence of Defects mean depth ofdefect, mm.

Heat sample 1,200 O. 1 hr., then bend. 0.45 many.

(Process 1).

1 Dependent on Cu content.

Example 3 With respect to the hot impact bend test of steel sample coated with tin and produced on purpose to form molten metallic substances the cause of the formation of surface defects at the temperature of hot working as described in the disclosure, the results obtained from the application of our invention are as follows:

Treatment Occurrence of Defects mean depth of defects, mm.

Heat sample 1,200 0., bend it immediately 0.5 many.

or after 5 min. (non-treated). Spray powdered mixture of iron sulphide ore none none.

60% and mill scale 40% on sample heat 1,200 O. 90 sec., then bend. (Process 2).

Spray powdered mixture of iron sulphide ore 40% and mill scale 60% on sample, melt powder, then bend. (Process 2).

Dip sample in the bath of iron ore containing S 20% and mill scale sec., then bend. (Process 1).

Example 4 In respect of the steam hammer hot forging of three steel ingots (diameter 60 mm., weight 2.5 kg.) containing copper 0.68%, 1.08%, and 2.0%, respectively, and tin 0.002%, to bars after heated in the furnace, the results accomplished by the application of our invention with the view of preventing surface defects are as follows:

Although several embodiments of the invention have been herein illustrated and described, it will be evident to those skilled in the art that various modifications may be made in the details of process and opcration'withont departing from the principles herein set forth.

We claim:

1. In hot-working a steel containing as impurity a. member selected from the group consisting of copper+tin+ arsenic and copper+tin; the improvement which comprises, in combination, applying to the bare surface of the aforesaid steel a material consisting essentially of an agent selected from the group consisting of iron sulfide, sodium sulfide, potassium sulfide, aluminum sulfide, mixtures of these sulfides with one another and mixtures of these sulfides with iron oxide and hot-working the resulting steel at a temperature at which the applied agent is in molten state; said agent being applied in an amount per unit area of said surface which is substantially the same as the amount of alloy of iron from the steel and said impurity present on said surface per unit area in liquid phase during the hot-working step.

2. The improvement as claimed in claim 1, wherein said agent is pulverulent.

3. The improvement as claimed in claim 1, wherein said agent is in the molten state.

4. The improvement as claimed in claim 1, wherein the steel contains from 0.18 to 2.0% by weight of copper as well as lesser amounts of tin.

5. The improvement as claimed in claim 1, wherein the steel contains from 0.18 to 2.0% of copper and from 0.02 to 0.5% of 6. The improvement as claimed in claim 1, wherein the steel contains from 0.18 to 2.0% of copper, from 0.02% to 0.5 of tin, and arsenic in lesser amounts than copper and at least in an amount of 0.06%.

7. The improvement as claimed in claim 5 wherein the steel further contains about 0.06% of arsenic.

8. The improvement as claimed in claim 1, wherein the temperature of the hot-working step is between about 940 and 1200 C.

9. The improvement as claimed in claim 1, wherein the temperature of the hot-working step is in the order of ll50l200 C.

10. The improvement as claimed in claim 1, wherein said agent is applied in the form of a molten bath into which said steel is dipped.

11. The improvement as claimed in claim said agent is applied in powder form.

12. The improvement as claimed in claim said hot-working is rolling.

13. The improvement as claimed in claim said hot-working is forging.

14. The improvement as claimed in claim 1, said hot-working is extruding.

15. The improvement as claimed in claim 1, wherein said agent consists essentially of a mixture of iron sulfide ore and mill scale.

16. In hot-working a steel containing as impurity a member selected from the group consisting of copper-*- tin+arsenic and copper+tin; the improvement of spraying the surface of the steel with a mixture consisting essentially of ferrous oxide and iron sulfide, and hotworking the resulting steel at a temperature at which said mixture is in molten state, said mixture being sprayed on in an amount per unit'area of said surface which is substantially the same as the amount of alloy of iron from the steel and said impurity in liquid phase present on said surface during the hot-working step.

17. In hot-working a steel containing as impurity a member selected from the group consisting of copper+ tin+arsenic and copper+tin; the improvement of dipping the steel into a bath of a molten mixture consisting essentially of ferrous oxide and iron sulfide, and hotworking the resulting steel at a temperature at which said mixture is in molten state, said mixture being applied to said surface in an amount per unit area of said surface which is substantially the same as the amount of alloy of iron from the steel and said impurity in liquid 1, wherein 1, wherein 1, wherein wherein step.

References Cited in the file of this patent UNITED STATES PATENTS Henricks June 7, 1955 Pakkala etal. May 17, 1955 Moore Apr. 9, 1957 8 FOREIGN PATENTS 755,923 Great Britain Aug. 29, '1956 OTHER REFERENCES The Makihg, Shaping and Treating of Steel, Fifth edition, published by Carnegie-Illinois Steel Corp. (United States Steel Corp. Subsidiary), 1940; p. 609.

Metals Handbook, 1948 edition; published by The Henricks July 22, 1958 10 American Society for Metals; pp. 7 and 25. 

1. IN HOT-WORKING A STEEL CONTAINING AS IMPURITY A MEMBER SELECTED FROM THE GROUP CONSISTING OF COPPER+TIN+ ARSENIC AND COPPER+TIN, THE IMPROVEMENT WHICH COMPRISES, IN COMBINATON, APPLYING TO THE BARE SURFACE OF THE AFORSAID STEEL A MATERIAL CONSISTING ESSENTIALLY OF AN AGENT SELECTED FROM THE GROUP CONSISTING OF IRON SULFIDE, SODIUM SULFIDE, POTASSIUM SULFIDE, ALUMINUM SULFIDE, MIXTURES OF THESE SULFIDES WITH ONE ANOTHER AND MIXTURES OF THESE SULFIDES WITH IRON OXIDE AND HOT-WORKING THE RESULTING STEEL AT A TEMPERATURE AT WHICH THE APPLIED AGENT IS IN MOLTEN STATE, SAID AGENT BEING APPLEID IN AN AMOUNT PER UNIT AREA OF SAID SURFACE WHICH IS SUBSTANTIALLY THE SAME AS THE AMOUNT OF ALLOY OF IRON FROM THE STEEL AND SAID INPURITY PRESENT ON SAID SURFACE PER UNIT AREA IN LIQUID PHASE DURING THE HOT-WORKING STEP. 